This invention relates to a process for recovering zinc and copper values from material containing significant quantities of zinc and copper oxides. More particularly, it relates to a hydrometallurgical process wherein greater than 90% of the copper and zinc present in the feed material is separated and recovered as acceptably pure metal.
In current zinc hydrometallurgical practice, zinc metal is recovered by electrowinning. Because of the relatively high position of zinc in the electromotive series, the concentration of metals such as cobalt, cadmium, nickel, copper, and iron in the zinc bearing solution must be quite low if an acceptably pure zinc product and a reasonable current efficiency are to be achieved. The presence of cobalt or nickel in the electrolyte lowers current efficiency in electrowinning and results in resolubilization of the cathode deposit; cadmium contaminates the zinc product; and iron, like cobalt and nickel, seriously lowers the current efficiency. To achieve reasonable economy in a zinc electrowinning system, the cobalt and nickel concentrations should be no greater than about 0.5 to 1.0 mg/l, the iron content should be less than about 20 mg/l, and cadmium should be present in concentrations no greater than about 50 mg/l.
The typical zinc-containing feed material used in the zinc hydrometallurgical-electrowinning process is a zinc oxide, iron containing calcine (e.g., ZnO.Fe.sub.2 O.sub.3) produced by roasting the raw ore and containing trace amounts of copper, cobalt, cadmium, and other metals. As disclosed, for example, in U.S. Pat. No. 3,985,857 to Menendez et al. entitled Process fo Recovering Zinc from Ferrites, and U.S. Pat. No. 3,994,721 to Bienvenu et al. entitled Purifying A Zinc-Bearing Solution by Cementation, the approach to winning zinc from this type of ore involves subjecting it to a hot acid leach to solubilize the metal values. After separating any acid insoluble residue from the leach liquor, the leach liquor is neutralized to a pH in the vicinity of 4-5 to precipitate impurities such as iron, nickel, and copper as insoluble hydrated oxides or hydroxides. This step is then followed by a cementation process wherein zinc metal dust is added to the leach liquor to precipitate metals remaining in solution which are more noble than zinc, such as cobalt and cadmium. The zinc values in the leach liquor may then be efficiently electrowon as an acceptably pure product in accordance with the reaction: EQU Zn.sup.++ +H.sub.2 O+SO.sub.4.sup.= .fwdarw.Zn.degree.+1/2O.sub.2 +H.sub.2 SO.sub.4
advantageously, the sulfuric acid produced as a byproduct may be used to solubilize additional quantities of roasted ore.
The foregoing technique is well suited for recovering zinc from ores containing quantities of copper too low to justify recovery. However, if materials of high copper concentration, e.g., brass factory dust, is sought to be treated, the foregoing technique is unsuitable because copper present in the leach liquor is precipitated as copper oxide or hydroxide together with the iron jarosites or hydroxides in the neutralization step and is lost or made more difficult to recover. An economical process for recovering zinc and copper from materials containing admixtures of copper and zinc oxide, iron values, and other metals must be capable of recovering substantial quantities of acceptably pure copper substantially uncontaminated by nickel, cobalt, iron, or zinc. Furthermore, the process should result in a zinc bearing leach liquor containing metallic contaminants no greater than the levels set forth above so that zinc cathodes of acceptable purity may be electrowon at reasonable current efficiencies.